A conventional electrostatographic copying process involves the formation of a latent electrostatic image on an element, typically an insulating substrate such as a photoconductor. Charged toner particles are then applied to the electrostatic image, where they adhere in proportion to the magnitude of the electrostatic potential difference between the toner particles and the charge on the image. The toner particles that form the developed image are then transferred to a receiver, usually electrostatically, by means of an electrostatic bias between the element and the receiver.
This conventional process works well with large toner particles. However, various difficulties arise when conventional electrostatic transfer techniques are used with smaller toner particles. Such particles are necessary to produce copies of very high resolution, because the granularity in an electrophotographic image is inversely proportional to the diameter of the toner particles used. As the size of the toner particles falls below about 8 micrometers, the surface forces holding the toner particles to the element tend to dominate over the electrostatic force that can be applied to the particles to assist their transfer to the receiver. Image quality, therefore, is reduced because less toner transfers. Moreover, those particles which do transfer frequently fail to transfer to positions on the receiver that are directly opposite their positions on the element. This "scattering" of toner particles due to repulsive coulombic forces lowers the resolution of the transferred image and increases graininess and mottle.
U S. Pat. No. 4,927,727 to Rimai et al. describes a thermally assisted method of transferring small toner particles which is designed to overcome the problems associated with electrostatic transfer. In this method, the receiver is heated, typically to about 60.degree. to 90.degree. C., and is pressed against the toner particles on an element. The heated receiver sinters the toner particles, causing them to stick to each other and to the receiver, thereby effecting the transfer of the toner from the element to the receiver. No electrostatic force is exerted on the toner particles during transfer. The temperature to which the receiver is heated is insufficient to melt or fix the toner particles, but is sufficient to fuse particles to each other at their points of contact.
To aid in transferring all of the toner particles from the element to the receiver, it is advantageous to coat the receiving surface of the receiver with a thermoplastic polymer. During transfer the toner particles adhere to or become partially embedded in the thermoplastic coating and thereby are more completely removed from the element. A further improvement in the procedure is to coat the thermoplastic polymer layer on the receiver with a low surface energy release agent. These improvements and preferred materials for the thermoplastic layer and the release agent are disclosed in more detail in U.S. Pat. No. 4,968,578 to Rimai et al., which is incorporated herein by reference.
While a release agent can advantageously be coated on the thermoplastic layer of the receiver sheet, other techniques can also be used to improve the transfer efficiency. For example, when the binder resin for the photoconductor and the thermoplastic polymer layer of the receiver are appropriately selected with respect to their compositions and surface energies, a release agent is not necessary. These improvements and examples of preferred materials are disclosed in U.S. Pat. Nos. 5,037,718 to Light et al., 5,043,242 to Light et al., and 5,045,424 to Rimai et al., which are incorporated herein by reference.
Even with the new thermally assisted transfer process disclosed in the cited patents, in each case some type of coated receiver is necessary to achieve the desired transfer efficiency, thereby increasing the cost of making high resolution copies using very small toner particles.
Accordingly, it would be desirable to provide a method for transferring very small toner particles from an element to a receiver which achieves complete or nearly complete toner transfer (and, thus, yields high quality, high resolution images) without the added costs, and complications, associated with special overcoat receivers. Additionally, it would be desirable to provide a small particle toner transfer method which achieves improved transfer efficiency when an overcoat receiver is used.